A chronic problem in the pacemaker art is that of minimizing the output energy of delivered stimulus pulses, either the ventricular pulse (VP) for a single chamber ventricular pacemaker, the atrial pulse (AP) for an atrial pacemaker, or both VP and AP for dual chamber pacemakers. Although battery sources have improved greatly in recent years, so too have capabilities for performing additional functions, such as obtaining and storing diagnostic data, telemetering such stored information or other pacemaker parameter data to an external programmer, etc. These new functions are enabled by dramatic advances in the area of data processing, particularly with the increased use of microprocessors and associated memory. Accordingly, whatever the battery capacity, the pacemaker manufacturer is constantly seeking additional ways to save energy so as to enable performance of additional functions while maintaining effective pacemaker lifetime. Thus, in designing future products it is more and more important to minimize the pulse output energy. This leads to the need for accurate measuring of evoked P and/or R waves, to determine whether or not a pace pulse has been effective in stimulating the heart. The technique of determining how low the output energy can be set is known as threshold searching, by which the threshold amount of energy needed to evoke cardiac response is determined. As is known in the art, once the threshold is determined, the pacing level can be set at some safe incremental level above threshold, to optimize the amount of energy delivered through the pace pulses.
The pacemaker patent literature reveals many different schemes for determining threshold to cardiac pacing. See, for example, U.S. Pat. Nos. 3,835,865; 4,305,396; 5,320,643; and 5,476,487. Most threshold searching arrangements rely on accurate sensing of the evoked response, i.e., determining whether the delivered pace pulse "captures" or evokes a cardiac response. However, a major problem has been to detect evoked potentials when they are superimposed on the polarization voltage which is generated at the electrode-tissue interface. Minimization of such polarization at the electrode where the pulse is delivered, and which follows a delivered pace pulse, is essential in measuring the evoked potential, or evoked response. If a large degree of polarization exists, it is difficult to detect an evoked response at the electrode. See U.S. Pat. No. 4,343,312, which provides an output circuit for delivering a triphasic pace pulse designed to minimize polarization and better enable detection of capture or no capture, as well as enabling detection of evoked T-waves. In such a triphasic pulse, pulses of opposite polarity are added both before and after the stimulus pulse, to counteract the buildup of charge at the electrode-tissue interface which would otherwise result from the stimulus pulse.
The present invention is based on the observation that reliable detection of evoked responses, as well as repolarization signals from the ventricle or atrium, requires a stable polarization environment. Thus, reliable capture detection can take place only when polarization has been minimized and stabilized, which condition should be achieved to the fullest possible extent before proceeding with threshold testing. It is accordingly an object of this invention to provide a system and method for capture detection and threshold determination, as well as for repolarization signal detection, which includes minimization and stabilization of polarization.